Classes

A class is a blueprint for your own kind of value: it says once what data every object made from it holds.

Last updated: 30 Sep, 2026 · Python 3.14

A dictionary accepts any key, so a typo adds a new key without any error. A class describes what every ticket has, and each ticket made from it is an object.

class Car:
    def __init__(self, window, door, enginetype):
        self.windows = window
        self.doors = door
        self.enginetype = enginetype

car1 = Car(4, 5, "petrol")
car2 = Car(3, 4, "diesel")
print(car1.windows)
print(car2.enginetype)

4
diesel

Each call to Car(...) ran __init__ with the three values, and each car kept its own. The Ticket class below is built the same way, with an id, a customer and a text.

Syntax:

class Name:
    def __init__(self, value):
        self.value = value     # an attribute

obj = Name(1)                  # runs __init__

Defining a Ticket class

class Ticket:
    def __init__(self, ticket_id, customer, text):
        self.id = ticket_id
        self.customer = customer
        self.text = text

ticket = Ticket(1, "Asha", "I was charged twice for one order")
print(ticket.customer)
print(ticket.text)

Asha
I was charged twice for one order

class Ticket: starts the description. By convention class names start with a capital letter.

__init__, with two underscores each side, is the function Python runs when you write Ticket(...). Its first parameter, self, is the new object being made; the lines inside store the arguments on it as attributes, read later with a dot.

You never pass self yourself. Ticket(1, "Asha", ...) gives three arguments, and Python supplies the object as the first.

Making many objects from one class

first = Ticket(1, "Asha", "I was charged twice for one order")
second = Ticket(2, "Ben", "My parcel has not arrived")

second.customer = "Ben T."
print(first.customer, "/", second.customer)

Asha / Ben T.

Each object keeps its own attributes. Changing second leaves first alone.

Reading an attribute that is not there

ticket = Ticket(1, "Asha", "I was charged twice for one order")
print(ticket.priority)

Traceback (most recent call last):
  File "main.py", line 2, in <module>
    print(ticket.priority)
AttributeError: 'Ticket' object has no attribute 'priority'

A dictionary would have said KeyError; an object says AttributeError. Same idea: you asked for something that was never stored. Delete that line before you add the next example: an error stops the file where it happens.

Printing an object

ticket = Ticket(1, "Asha", "I was charged twice for one order")
print(ticket)

<__main__.Ticket object at 0x10a240d70>

Python does not know which attributes matter, so it prints the class name and a memory address, which changes on every run. dataclasses fixes that.

Dictionary vs class

Where classes show up in AI code

• Frameworks hand you classes to create: a client, an agent, a model.
• Your own records, such as a ticket or a result, with fixed fields.

Related

• Previous: try and except
• Next: Methods
• Reference: Classes in the Python tutorial